Magnetite (Fe3O4) is an attractive anode material for lithium-ion batteries (LIB) based on the conversion mechanism according to the reaction:Fe3O4+8Li++8e−3Fe+4Li2O.
Fe3O4 is characterized by a theoretical capacity of 926 mAhg−1, which is far beyond that of the graphite anode (372 mAhg−1), eco-friendliness, natural abundance and high electronic conductivity (2×104 Sm−1). Its application in practical lithium-ion batteries (LIB) is, however, hindered due to its low rate performance arising from kinetic limitations, agglomerations and poor cycling stability resulting from significant volume expansion during the redox reaction.
To overcome these problems, Banov et al. (B. Banov, L. Ljutzkanov, I. Dimitrov, A. Trifonova, H. Vasilchina, A. Aleksandrova, A. Mochilov, B. T. Hang, S. Okada and J. I. Yamaki, 2008, J. Nanosci. Nanotech, 591-94) described the synthesis of a Fe3O4 nanocomposite on a carbon matrix by using a thermal decomposition of iron nitrate. This composite encompasses carbon particles with an average size of about 100-150 nm which are covered by small Fe3O4 crystallites of a mean size of 16 nm. The modified material yielded a specific capacity of about 400 mAh/g after 30 cycles as an anode in lithium batteries. The synthesis was, however, achieved only by several successive steps including the preparation of the carbon support.
Liu et al. (H. Liu, G. Wang, J. Wang and D. Wexler, 2008, Electrochem. Commun. 1879-82) described a synthesis of carbon coated magnetite (Fe3O4) core-shell by a multi-step hydrothermal method yielding Fe2O3 nanorods which were sintered together with citric acid as carbon source. The resulting carbon coated Fe3O4 nanorods have a diameter in the range of 30-50 nm and a length extending a few 100 nanometers. The prepared Fe3O4/C core-shell nanorods exhibited an initial lithium storage capacity of 1120 mAh/g and a reversible capacity of 394 mAh/g after 100 cycles.
Muraliganth et al. (T. Muraliganth, A. V. Murugan and A. Manthiram, 2009, Chem. Commun., 7360-62) described a carbon-coating of Fe3O4-nanowires with glucose-derived carbon-rich polysaccharide in a two-step process by sonication and heating at 400° C. for 3 h. TFe3O4-nanowires are, however, complex to synthesize by a time and energy consuming procedure. The use of the Fe3O4-nanoparticles was not disclosed.
Wang et al. (S. Wang, J. Zhang and C. Chen, 2010, J. Power Sources, 195, 5379-81) described synthesized uniform submicron spheroids of Fe3O4 by a hydrothermal method. This material has been studied for its capacity as electrode material in Li-ion batteries. While having initially good characteristics, the material lacks stability during long term use, in particular due to the volume changes and agglomeration of the particles.
In another described example, a Fe3O4-graphene (23 wt.-%) nanocomposite was prepared by a gas/liquid interface reaction (P. Lian, X. Zhu, H. Xiang, Z. Li, W. Yang and H. Wang, 2010, Electrochim. Acta, 834-40). The nanocomposite exhibited good capacity retention (99% after 90 cycles) with a large reversible capacity of 1048 mAh/g. The nanocomposite synthesis is, however, a very complicated, multistep and time consuming process, which required several work-up steps starting from iron nitrate and graphene sheets.
Song et al. (S. Song, R. Rao, H. Yang, H. Liu and A. Zhang, 2010, Nanotechnology, 185602(6pp)) described a fabrication process of Fe3O4 nanoparticles with a size range of 4-8 nm by a spontaneous redox reaction between Fe3+ and multi-walled carbon nanotubes (MWCNTs). In this process, a multi-step pre-conditioning of the MWCNT is required. No electrochemical studies were moreover performed.
He at al. (H. He, L. Huang, J. S. Cai, X. M. Zheng and S. G. Sun, 2010, Electrochim. Acta 55, 1140-44) described the synthesis and electrochemical performance of a nanostructured Fe3O4/carbon nanotube composite. In this complicated multi-step synthesis process, polyvinyl alcohol (PVA) was used as a hydrogen bond functionalizing agent to modify the multi-walled CNTs. The nanoparticles of Fe3O4 were then deposited along the sidewalls of the as modified CNTs, the chemical coprecipitation of (NH4)2Fe(SO4).6H2O and NH4Fe(SO4).12H2O, in the presence of CNTs in an alkaline solution. The final product was obtained by several purification steps, including removal of ammonia, sulphate, unreacted, PVA etc.
Zhang et al. (M. Zhang, D. Lei, X. Yin, L. Chen, Q. Li, Y. Wang and T. Wang, 2010, J. Mater. Chem. 20, 5538-43) described the synthesis of magnetite/graphene composites by a microwave irradiation method. In this method, graphene oxide (GO) was first prepared from graphite, then exfoliation was carried out by sonication of the GO. In the next step, the mixture of GO, Fe(NO3)3, urea and ascorbic acid was refluxed under ambient condition for 1 h with a microwave heater. The method additionally requires several purification and heat treatment steps. The modified electrode material exhibited a reversible capacity of 650 mAh/g after 50 cycles.
Zhou at al. (G. Zhou, D.-W. Wang, F. Li, L. Zhang, N. Li, Z.-S. Wu, L. Wen, G. Q. Lu and H.-M. Cheng, 2010, Chem. Mater. 22, 5306-13) subsequently described the synthesis and electrochemical properties of a graphene-wrapped Fe3O4 composite. The composite was synthesized by dispersing a graphene nanosheet and FeCl3 in an aqueous solution by sonication, followed by hydrolysis to obtain a FeOOH/graphene sheet. After several purification steps, the resulting product was then reduced to give the desired nanocomposite by heat treatment under inert condition. This material was tested as an anode in lithium batteries and obtained a specific capacitiy of 1026 mAh/g after 30 cycles at a given current density of 35 mA/g.
Ban et al. (C. Ban, Z. Wu, D. T. Gillaspie, L. Chen, Y. Yan, J. L. Blackbrurn and A. C. Dillon, 2010, Adv. Mater. 22, E145-49) recently described high-rate capability binder-free electrode materials consisting of Fe3O4 and single walled carbon nanotubes (SWCNTs). The best performance was obtained by the nanocomposite containing 95 wt.-% oxide and 5 wt.-% SWCNT. The nanocomposite was prepared by a two-step process, hydrothermal synthesis and vacuum filtration. The most crucial step is the purification/deagglomeration of SWCNTs, which is time consuming. SWCNTs were additionally synthesized by a laser vaporization technique. Both the material and method are very expensive.
Another solution for circumventing the problems of volume changes and agglomeration of Fe3O4 as an anode material was described by Ji et al. (L. Ji, Z. Tan, T. Kuykendall, S. Aloni, S. Xun, E. Lin, V. Battaglia and Y. Zhang, 2011, Phys. Chem. Chem. Phys. 13, 7170-77) where a nanocomposite of reduced-graphene-oxide (RGO) in which Fe3O4 nanoparticles are encapsulated was described. The Fe3O4 nanoparticles are uniformly anchored in the RGO sheets such that severe volume changes or agglomeration of the particles can be avoided. The material was tested as an anode material in Lithium ion batteries. The synthesis of the Fe3O4-RGO nanocomposite is a time consuming and a multi-step process.
Wang et al. (J.-Z. Wang, C. Zhong, D. Wexler, N. H. Idris, Z.-X. Wang, L.-Q. Chen, H.-K. Liu, 2011, Chem. Eur. J., 17, 661-667) described the preparation of a nanocomposite consisting of Fe3O4 particles embedded in graphene nanosheets. The nanocomposite was tested as an electrode material in a lithium ion battery where it exposed significantly improved life cycles compared to pure Fe3O4. The nanocomposite was synthesized in situ starting from graphite and FeCl2 by a multistep hydrothermal method.
The preparation of a further Fe3O4 nanocomposite in form of hierarchically structured Fe3O4/carbon micro-flowers was described by Jin et al. (S. Jin, H. Deng, D. Long, X. Liu, L. Zhan, X. Liang, W. Qiao, L. Ling, 2011, J. Power Sources, 196, 3887-3893). This composite consists of nanosized Fe3O4 crystallites and amorphous carbon which are assembled to micro flowers of 2-5 μm. The paper also discloses a method of a controlled thermal decomposition of an iron-alkoxide precursor for preparing said nanocomposite. This preparation requires energy and time consuming efforts. The Fe3O4 nanocomposite is proposed and assayed for its capability as an anode material in lithium ion batteries.
A common disadvantage of the state of the art is the lack of an economic one-step solvent-free synthesis which is easy to perform. The cited state of the art does not disclose any nanoparticles or nanocomposites consisting of a transition metal oxide, in particular Fe3O4, which are encapsulated in a carbon shell.